Anchor alarm radius: how to set one that will not cry wolf

In this piece

1. What a GPS anchor alarm actually watches
2. How to work out the anchor alarm radius
3. Where you set the anchor marker matters more than the radius
4. Phone GPS accuracy on a boat
5. Too tight cries wolf, too wide lets you drag
6. Wind shifts and tide
7. Circles, arcs and rectangles
8. Why a phone alarm does not have to cry wolf

Set your anchor alarm radius to the horizontal reach of the chain, plus the distance from the bow to wherever the phone is, plus about 10 m for GPS error and the boat sheering about on her chain. With 40 m of chain out in 8 m of water on a 12 m (40 ft) boat, that comes to about 60 m (200 ft).

Then make sure the centre of the circle is on the anchor. A sensible radius around the wrong point is the usual reason an anchor alarm app goes off over a perfectly good anchor, and the usual reason it stays quiet while the boat drags.

What a GPS anchor alarm actually watches

An anchor alarm on a phone knows one thing: where the phone is. It does not know where the anchor lies, where the bow is or how much chain you have out. It draws a circle around a point you give it and sounds when the phone leaves that circle.

And the phone is nowhere near the anchor. It is in the cockpit, on the chart table or beside your bunk, up to a boat length aft of the bow roller, and the bow roller is a chain length from the anchor. The radius is those two distances added together, with something on top for the phone’s own uncertainty.

How to work out the anchor alarm radius

Take a 12 m boat anchored in 8 m of water on 40 m of chain, with the bow roller 1.2 m above the surface.

The chain’s horizontal reach is a little less than its length, because some of it is spent going down. Stretched straight from a 1.2 m roller to the bottom in 8 m of water, 40 m of chain reaches about 39 m across the seabed. You can use the chain length and be within a metre or two; the depth hardly changes the answer.

For the phone, use the whole boat length unless you know it lives well forward. That gives a few metres of slack when the phone is in the saloon, and a phone in the cockpit or an aft cabin sits at the end of the boat that swings about most when she sheers in the gusts.

Where you set the anchor marker matters more than the radius

This is the mistake nearly everyone makes once. You anchor, fall back, set the anchor, stop the engine, tidy the lines, and only then think of the alarm. You mark the anchor from the cockpit, and the app puts it where the phone is: some 50 m downwind of where it lies.

Two things now go wrong. When the wind shifts in the night and the boat swings to the far side of the real anchor, she ends up about 100 m from the marker, and a 60 m alarm goes off over an anchor that has not moved. Worse, if the anchor drags straight back, the boat has to travel the full 60 m from where she started before anything sounds. With the marker on the anchor, the same drag would have set it off after about 10 m.

There are two ways to get it right. The simplest is to mark the anchor as it goes down. With the phone at the bow, the marker lands on the anchor. With the phone at the helm, it lands about a boat length downwind of it, which is a small error but worth fixing: drag the marker a boat length upwind, or add the boat length to your margin.

The other is to put the marker where the anchor is after the fact. You know how much chain is out, and with the chain taut the bow points roughly at the anchor. Drop the marker that far ahead of the boat along the line of the bow and you will be within a few metres. An hour later the boat’s track will tell you whether you got it right. An anchored boat draws arcs, and the anchor sits at the centre of the curve. If the marker is off to one side of it, move it.

Phone GPS accuracy on a boat

Out in the open, a modern phone usually knows where it is to within a few metres. Below decks it does worse. A fibreglass deck costs some accuracy, and a steel or aluminium hull can block the signal almost completely. The mast, the rigging and a stack of electronics round the chart table bounce the signals about, which makes the reported position wander while the boat sits still.

Phones attach their own accuracy estimate to every fix. If the phone does well in the cockpit and badly on the chart table, the chart table is the wrong place for it. A spot near the companionway or under a hatch is usually good enough. Keep it out of direct sun on a summer afternoon, though: phones shut down when they overheat, and an alarm on a phone that has switched itself off is watching nothing.

That is what the margin is for. We use about 10 m when the phone has a good spot and 15 m when it has to live below. Much less and an ordinary bad fix sets it off; much more and you are giving away room you may not have.

Too tight cries wolf, too wide lets you drag

A radius smaller than the swing goes off as soon as a gust stretches the chain out, or the wind veers and the boat swings across. A 30 m circle with 40 m of chain out will do it on the first proper gust. After two or three false alarms in one night most people widen the circle a long way or switch the thing off, and that is the real cost of a tight radius.

A radius that is too wide fails the other way. If your 60 m sum becomes 120 m “to be safe”, the boat can drag 60 m before anything happens. Dragging at one knot, a boat covers about 30 m a minute, and in a strong wind she can go faster than that. Measure from the edge of your circle to the nearest thing you could hit, in any direction tonight’s wind might come from, and divide by 30. That is roughly how many minutes you have between the alarm and the bump, and what to do when your anchor drags is how to spend them.

If that comes out at less than a couple of minutes, the trouble is the spot: the anchorage is too tight for the chain you have out. Move, or change the shape of the zone.

Wind shifts and tide

A circle centred on the anchor copes with a wind shift by design. The boat swings to the other side of the anchor and stays the same distance from it. That is also why the centre has to be right: a wrong centre stays hidden until the wind goes round, and then it looks exactly like dragging. On many coasts a land breeze comes down in the night from roughly the opposite direction to the afternoon’s wind, so assume you will use the whole circle.

In tidal waters the boat swings to the other side every time the stream turns, roughly every six hours, so the whole circle gets used whatever the wind does. The changing depth makes little difference to the radius: our 40 m of chain reaches about 39.7 m across the bottom in 4 m of water and about 37.8 m in 12 m. It makes a big difference to scope, so work out the chain for high water first; how much anchor chain to let out has the table.

Circles, arcs and rectangles

A plain circle suits most anchorages. It suits two common situations badly: rocks close on one side with open water on the other, and a boat that is not swinging at all because she is tied stern-to or alongside.

SafeBoat’s anchor alarm has three zone modes, one for each case. Radius is the plain circle, from 1 to 300 m (3 to 900 ft); the default is 30 m, so with any real length of chain out you will want to widen it. Radius Pro puts an outer arc around an inner circle. The inner circle is always safe, and the arc reaches further on the side you choose, up to 500 m out and at least 5 m beyond the inner circle. Point it at open water and you can keep the alarm tight toward the rocks and generous where a little extra swing costs nothing; the arc can also go all the way round to a full circle. Area is a rectangle, 4 to 400 m a side and 20 by 40 m by default, which you rotate to fit a berth when the boat is tied up stern-to or alongside. You reshape any of them by dragging the handles on the map, and the anchor marker drags too. There is no chain length or bow offset to type in. You do the sum above and draw the result.

Why a phone alarm does not have to cry wolf

Every phone produces the odd bad fix: a reflection off the mast, a moment with fewer satellites in view, a position that jumps 40 m and comes back. An alarm that believes every fix will wake you at 3 am for nothing.

Whatever anchor watch app you use, test it before you need it. On a quiet evening in harbour, set a small radius, walk the phone up the pontoon until it goes off, and check that you can hear it from your bunk with the cabin door shut. Then, at anchor, set it properly: the sum above, and the marker on the anchor. The rest of the night’s routine is in sleeping well at anchor.